The objectives of this research are the development and genetic characterization of sets of recombinant inbred (RI) strains of mice, which were derived from the crosses of pairs of inbred (progenitor) strains. Thses RI strains will be used to genetically analyze numerous differences between the progenitors, including leukemia susceptibility. The RI strains will be typed for newly discovered polymorphisms, which will serve as genetic markers for the analysis of inheritance of complex traits such as susceptibility to leukemia and autoimmunity. RI strains constitute a powerful genetic method to identify, map, and study the mechanisms of action of individual genes. These strains will be made available to other investigators. As these strains become increasingly will characterized genetically, their importance as a genetic resource will be enhanced. Freeze-preservation of embryos will be employed to assure the longterm survival of these strains. The leukemia incidence and latency will be determined for 26 AKXD RI strains which were inbred from the cross of the high leukemia strain AKR/J with the low leukemia DBA/2J stain. Possible synergism between an endogenous ecotropic murine leukemia virus and Gama-irradiation on the incidence and latency of reticulum cell sarcomas in strain C57L/J will be investigated.